I. Field of the Invention
This invention relates to the casting of composite metal ingots by sequential direct chill casting. More particularly, the invention relates to such casting in which compensation is made for variations of the input temperatures of molten metals being cast.
II. Background Art
It is desirable for many purposes to cast metal ingots made of two or more metal layers. For example, rolled products produced from such ingots may be formed with a metal coating layer on one or both sides of a core layer in order to provide specific surface properties that may differ from the bulk properties of the metal product. A very desirable way in which such composite ingots may be cast is disclosed in International Patent publication no. WO 2004/112992 naming Anderson et al. as inventors. This publication discloses a method of, and apparatus for, direct chill (DC) casting two or more metal layers at one time to form a composite ingot. For good adhesion between the metal layers, it is desirable to ensure that the layers, while being cast together in a single apparatus, are formed sequentially so that molten metal of one layer contacts previously-cast semi-solid metal of another layer, thereby allowing a degree of metal co-diffusion across the metal-metal interface(s). The casting arrangement may also prevent undue oxide formation at the interface(s) between the metal layers, again improving mutual adhesion of the layers.
It has been found by the inventors named herein that the temperatures of the molten metals used for the casting of various layers can affect the operation of the casting method and apparatus. If one or more of the metal streams is too hot, rupture or other kind of failure of the metal-metal interface where the metals first come into contact may occur as the ingot is being formed. On the other hand, if one or more of the metal streams is too cold, the flow of molten metal into the casting mold can be hindered due to partial or complete freezing of the metal in downspouts or distribution troughs used for conveying the metals to the casting mold. Additionally, in such cases, pre-solidified material may be delivered to the casting mold itself which adversely affects the cast product. While the apparatus is generally optimized to deliver metals to the mold at desired temperatures (referred to as a “set point” for a particular metal), it is not always easy in practice to maintain the desired temperatures due to environmental factors and unexpected operational variations. It is therefore desirable to provide a way of negating or minimizing the adverse effects of such temperature variations.
While the above-mentioned International patent publication to Anderson et al. discloses a basic process for co-casting multiple layers to form composite ingots, the problems caused by variations of input temperatures are not discussed or disclosed and no solutions are discussed.
U.S. Pat. No. 5,839,500 to Roder et al. issued on Nov. 24, 1998 discloses a method and apparatus for casting a metal slab by a continuous process involving the use of a twin belt caster, moving block caster, or the like. The patent suggests ways of improving the quality of metal castings involving measuring such things as metal temperatures and controlling certain process parameters. However, the patent is not concerned with casting composite ingots and does not involve the supply of two or more metal streams to a casting apparatus.
There is therefore a need for ways of effectively addressing some or all of the problems mentioned above.